This invention relates to paper comprising aldehyde modified cellulose pulp or fiber and further to the method of preparing aldehyde modified cellulose and cellulose pulp or fiber using selected oxidation conditions to generate aldehyde functionality. More particularly, this invention involves paper made from cellulose pulp having a defined amount of aldehyde content. A method for preparing the selected aldehyde modified cellulose and cellulose pulp involves using a nitroxyl radical mediated oxidation with a limited amount of oxidant and defined reaction conditions. This aldehyde modified pulp is used in the production of tissue/towel and other paper products which exhibit unexpected high wet strength, temporary wet strength and dry strength properties, including compression strength and resistance, and high wet strength/dry strength ratios without the use of other additives.
The term xe2x80x9cpaperxe2x80x9d as used herein, includes sheet-like masses and molded products made from pulp or fibrous cellulosic material which may be derived from natural sources. Paper may also be made from synthetic cellulosic fibers and regenerated cellulose as well as recycled waste paper. In addition, paper made from combinations of cellulosic and synthetic materials are applicable herein. Paperboard is included within the broad term xe2x80x9cpaperxe2x80x9d.
Papermaking, as it is conventionally known, is a process of introducing an aqueous slurry of pulp or wood cellulosic fibers (which have been beaten or refined to achieve a level of fiber hydration and to which a variety of functional additives can be added) onto a screen or similar device in such a manner that water is removed, thereby forming a sheet of the consolidated fibers, which upon pressing and drying can be processed into dry roll or sheet form. Typically in papermaking, the feed or inlet to a papermaking machine is an aqueous slurry or water suspension of pulp fibers which is provided from what is called the xe2x80x9cwet endxe2x80x9d system. In the wet end, the pulp along with other additives are mixed in an aqueous slurry and subject to mechanical and other operations such as beating and refining. Various additives are commonly added to help provide different properties in the paper product.
The preparation of aldehyde containing starches and the use of such aldehyde derivatives in the paper industry as wet and dry strength additives is well known. Both oxidative and non-oxidative methods are known for introducing aldehyde groups into starch. Use of these products in papermaking to provide wet and dry strength properties involves the addition of this separate starch additive component.
The use of nitroxyl radicals and nitrosonium salts in organic chemistry as an oxidative route to produce aldehydes and carboxylic acids from primary and secondary alcohols is disclosed in an article entitled xe2x80x9cOrganic Nitrosonium Salts As Oxidants in Organic Chemistryxe2x80x9d by J. M. Bobbitt and C. L. Flores, in Heterocycles, Vol. 27, No. 2, 1988, pp. 509-533. Recently, application of this chemistry was extended to the selective oxidation of primary alcohols in various carbohydrates to carboxylic acids in an article entitled xe2x80x9cSelective Oxidation of Primary Alcohols Mediated by Nitroxyl Radical in Aqueous Solution. Kinetics and Mechanismxe2x80x9d by A. E. J. de Nooy and A. C. Bessemer, in Tetrahedron, Vol. 51, No. 29, 1995, pp. 8023-8032. Patent publication WO 95/07303 dated Mar. 16, 1995 further discloses the use of this technology where carbohydrates having a primary hydroxyl group are oxidized under aqueous conditions to form products having a high content of greater than 90% carboxyl groups. This art involving the oxidation of primary alcohols generally describes the preparation of polyglucuronic acids with high carboxylic acid content. Similarly, the process of oxidation has been used to prepare various polysaccharides with high carboxyl content as described in xe2x80x9cOxidation of Primary Alcohol Groups of Naturally Occurring Polysaccharides with 2,2,6,6-Tetramethyl-1-piperidine Oxoammonium Ionxe2x80x9d by P. S. Chang and J. F. Robyt in J. Carbohydrate Chemistry, 15(7), 1996, pp. 819-830. It should be noted that in some applications high carboxylic acid content is undesirable.
Recent patent publications WO 99/23240 and 99/23117, both dated May 14, 1999, respectively discose methods of oxidizing starch and cellulose using an oxoammonium ion producing reagent in the presence of an enzyme oxidizing agent.
Despite the various methods described above, there still is the need for cellulose pulp which is suitable for use in paper applications to provide the desired high degree of wet strength, temporary wet strength and dry strength properties, and improved compression strength and resistance, and does not involve the use of separate additive components.
This invention is directed to paper having wet strength, temporary wet strength and dry strength properties, including improved compression strength and resistance, and comprising aldehyde modified cellulose pulp wherein the pulp has from 1 to 20 mmoles of aldehyde per 100 g of cellulose.
Another embodiment of this invention involves the selective preparation of cellulose aldehyde and cellulose pulp aldehyde under defined oxidation conditions using a nitroxyl radical mediated aqueous oxidation procedure to provide derivatives with effective aldehyde content particularly useful in papermaking. More particularly, this invention involves the oxidation of cellulose or cellulose pulp in an aqueous solution with an oxidant having an equivalent oxidizing power of up to 5.0 g of active chlorine per 100 g of cellulose and an effective mediating amount of nitroxyl radical, the reaction being carried out at a pH of about 8.0 to 10.5, and a temperature of from about 5 to 50xc2x0 C., the resulting product having an aldehyde content of about 1 to 20 mmole/100 g of pulp.
This invention further involves aldehyde modified cellulose or cellulose pulp having defined aldehyde content.
Still another embodiment involves the method of preparing paper having wet strength, temporary wet strength and dry strength properties, including compression strength and resistance, comprising using the cellulose aldehyde pulp prepared by the selective oxidation procedure as described above, as the paper or pulp stock or a component thereof.
The cellulose or cellulose pulp aldehyde derivatives of this invention have effective aldehyde functionality or content of from about 1 to 20 and preferably from about 5 to 20 mmoles/100 g of cellulose material, i.e., cellulose or cellulose pulp.
The cellulose aldehyde derivatives of this invention can be prepared by a method which involves the selective oxidation of cellulose and cellulose pulp or fiber using a limited amount of oxidant mediated with a nitroxyl radical under defined conditions to provide derivatives with effective aldehyde content making them particularly suitable for use in providing paper with desired wet strength, temporary wet strength and dry strength properties, induding compression strength and resistance.
The nitroxyl radical mediator used herein is a di-tertiary alkyl nitroxyl radical having one of the following formulas: 
where A represents a chain of preferably two or three atoms, in particular carbon atoms or a combination of one or two carbon atoms with an oxygen or nitrogen atom, and the R groups represent the same or different alkyl groups. Chain A may be substituted by one or more groups such as alkyl, alkoxy, aryl, aryloxy, amino, amido or oxo groups, or by a divalent group or multivalent group which is bound to one or more other groups having formula I. Particularly useful nitroxyl radicals are di-tertiary alkyl nitroxyl radicals having the formula: 
where Y is either H, OH or NHxe2x80x94C(O)xe2x80x94CH3 and each of the R groups represent the same or different alkyl groups of 1 to 18 carbon atom and more particularly methyl groups. Nitroxyl radicals of this type include those where a) the R groups are all methyl (or alkyl of 1 carbon atom) and Y is H, i.e., 2,2,6,6-tetramethyl-1-piperdinyloxy (TEMPO); b) R groups are methyl and Y is OH and identified as 4-hydroxy TEMPO; and c) R groups are methyl and Y is NHxe2x80x94C(O)xe2x80x94CH3 and identified as 4-acetarido-TEMPO. The preferred nitroxyl radical is TEMPO or 4-acetamido-TEMPO. The nitroxyl radical is used in an effective amount to mediate the oxidation and more particularly from about 0.001 to 20% by weight, preferably from about 0.01 to 0.1% by weight, based on the weight of cellulose, cellulose pulp or fiber. The nitroxyl radical can be added to the reaction mixture or generated in situ from the corresponding hydroxylamine or oxoammonium ion.
The oxidant used in this invention can be any material capable of converting nitroxyl radicals to their corresponding oxoammonium salt. Particularly useful oxidants are the alkali or alkaline-earth metal hypohalite salts such as sodium hypochlorite, lithium hypochlorite, potassium hypochlorite or calcium hypochlorite. An alkali or alkaline earth-metal hypobromite salt may also be used and it may be added in the form of the hypobromite salt itself, such as sodium hypobromite, or it may be formed in situ from the addition of a suitable oxidant such as sodium hypochlorite and an alkali or alkaline-earth metal bromide salt such as sodium bromide. The bromide ion is generally in the form of sodium bromide. Additional oxidants that can be used in this method include hydrogen peroxide in combination with a transition metal catalyst such as methyltrioxorhenium (VII); hydrogen peroxide in combination with an enzyme; oxygen in combination with a transition metal catalyst; oxygen in combination with an enzyme; peroxyacids such as peracetic acid and 3-chloroperoxybenzoic acid; alkali or alkaline-earth metal salts of persulfates such as potassium persulfate and sodium persulfate; alkali or alkaline-earth metal salts of peroxymonosuffates such as potassium peroxymonosulfate; chloramines such as 1,3,5-trichloro-1,3,5-triazine-2,4,6(1H,3H,5H)trione, 1,3-dichloro-1,3,5-triazine-2,4,6(1H,3H,5H)triione sodium salt, 1,3-dichloro-5,5-dimethylhydrantoin, 1-bromo-3-chloro-5,5-dimethylhydrantoin, and 1-chloro-2,5-pyrrolidinedione; and alkali or alkaline-earth metal salts of ferricyanide. This list of oxidants is only illustrative and is not intended to be exhaustive. The oxidants can be used alone or in combination with an alkali or alkaline-arth metal bromide salt. The preferred oxidant is sodium hypochlorite or sodium hypobromite formed from the addition of sodium hypochlorite and sodium bromide.
The important factor in the use of the oxidant is that it must be used in a limited amount that has the equivalent oxidizing power of up to 5.0 g of active chlorine per 100 g of cellulose or cellulose pulp. More particularly, the amount of oxidant used will have an equivalent oxidizing power of from about 0.05 to 5.0 g of active chlorine and preferably from about 0.5 to 2.5 g of active chlorine per 100 g of cellulose or cellulose pulp. When sodium hypochlodte is used, it is used in a limited amount of up to about 10 percent by weight based on the weight of cellulose or cellulose pulp, more particularly from about 0.1 to 10% and preferably from about 1 to 5% by weight based on the weight of cellulose or cellulose pulp. Bromide in the form of sodium bromide will generally be used in an amount of from about 0.1 to 5% by weight and preferably from about 0.25 to 2% by weight based on the weight of cellulose or cellulose pulp. By limiting the amount of oxidant under defined aqueous conditions, the cellulose aldehyde derivatives are selectively prepared at effective high aldehyde levels. Such high aldehyde cellulose products are particularly useful in preparing paper with wet strength, temporary wet strength and dry strength properties, including compression strength and resistance.
The cellulose material used as the starting material may be any cellulose, cellulosic fiber or pulp material. This includes hardwood or softwood cellulosic fibers such as bleached and unbleached sulfate (Kraft), bleached and unbleached sulfite, bleached and unbleached soda, neutral sulfite, semi-chemical, groundwood, chemi-groundwood, and any combination of these fibers. In addition, synthetic cellulosic fibers of the viscose rayon or regenerated cellulose type can also be used, as well as recycled waste papers from various sources. The consistency in water of the cellulose or pulp that is used will be from about 0.1 to 15% by weight solids in water and preferably from about 1 to 5% by weight. When used in papenmaking other additives such as desired inert fillers or retention aids may be added to the cellulose pulp. Such materials include clay, titanium dioxide, talc, calcium carbonate, calcium sulfate and diatomaceous earth. Rosin or synthetic internal size may also be present, if desired. Other additives commonly used in paper may also be used in combination with the oxidized pulp of this invention.
The oxidation reaction of the cellulosic material is carried out in an aqueous solution. The pH of the reaction is maintained at about 8.0 to 10.5, preferably about 9 to 10, the temperature is maintained at from about 5 to 50xc2x0 C., preferably from about 20 to 30xc2x0 C. The extent of the reaction is controlled by the amount of oxidant used or the reaction time. Generally the reaction time will be from about 5 to 60 minutes, and more particularly from about 20 to 30 minutes.
By using the reagent and component amounts as defined previously and the noted reaction conditions, controlled amounts of aldehyde functionality, particularly C-6 aldehyde, can be obtained that are suitable and effective in providing desired wet strength, temporary wet strength, and dry strength properties, including compression strength and resistance, and wet strength/dry strength ratios desired in the final prepared paper product. The cellulose aldehyde derivatives prepared in accordance with this invention will have effective aldehyde functionality of from about 1 to 20 and preferably from about 5 to 20 mmole/100 g of cellulosic material i.e., cellulose or cellulose pulp. Carboxylic acid functionality will also be generated or formed during the oxidation process. Amounts of carboxyl content generated will generally be from about 1 to 40 mmole/100 g of cellulose or cellulose pulp, particularly from about 1 to 20 and more particularly from about 1 to 10 mmole/100 g cellulose or cellulose pulp. It should be noted that this amount of carboxylic acid functionality is in addition to what may already be present in the cellulose or cellulose pulp naturally or by virtue of the type of processed pulp used, such as bleached sulfate, bleached sulfite, etc. The effective level of aldehyde is an important aspect of this invention and one way this can be defined is by the ratio of aldehyde to generated carboxylic acid functionalities. Such levels can be defined by aldehyde to generated carboxylic acid ratios of greater than or equal to 0.5 (based on mmole/100 g of cellulose or cellulose pulp of each functionality) and preferably greater than or equal to 1.0. While recognizing that the amount of additional carboxylic functionality (i.e., other than generated) will vary and may be fairly low, there nevertheless will be some present and this will affect the level of total carboxylic acid functionality. Considering this and based on total carboxylic acid, the ratio of aldehyde to carboxylic acid functionality will be from about 0.2 or more. The significance of this aldehyde content is particularly manifested in the resulting properties found in paper prepared from the oxidized cellulose material. High wet strength, temporary wet strength and dry strength properties, including compression strength and resistance, are found. Products having high wet strength/dry strength ratios of greater than 20% have been obtained in paper using these selectively modified cellulose aldehyde derivatives indicating improved properties such as softness.
Compression strength and resistance are components of the dry strength of paper, and are particularly significant in paperboard quality. Compression strength relates to the edgewise compression strength of paper and is commonly measured by the Swedish Forest Products Laboratory (STFI) short-span test. Compression resistance is a measure of the elastic and inelastic buckling or edge crush resistance of the paper and is commonly measured by the Ring Crush test. The Ring Crush and STFI tests are described in detail in the Handbook of Physical and Mechanical Testing of Pulp and Paperboard, Johnson, J. A., Bennett, K. A. et al, Marcel Decker, N.Y., (1983) pages 181-190, which is incorporated herein by reference. The modified papers of the present invention demonstrate improved compression properties as expressed by their STFI and Ring Crush tests of greater than about one percent, and preferably greater than about five percent, over papers made from a corresponding unmodified pulp.
Further, the moisture absorption by papers which is caused by high humidity conditions is known to significantly reduce the compression strength of papers. The papers of the present invention demonstrate improved compression strength and resistance even in environments of relatively high temperature and humidity. This increase in compression strength is greater than about 1%, preferably greater than about 5% over the compression strength of papers made from a corresponding unmodified pulp.
It is noted that use of the modified aldehyde cellulose derivatives of this invention in papermaking may involve the use of such derivatives as the whole or entire pulp or paper stock or it may be used as a component of the paper stock (i.e., in amounts of 20, 40, 60% by weight etc.).